In a variety of mechanical or structural devices or assemblies including, for example, aircraft, spacecraft and other types of vehicles or vessels; buildings; bridges and other structures, it is frequently desired to be able to determine changes in material properties of a given part, because such changes can be indicative of degradation of the part. For example, material stiffness is an important parameter affecting the performance of a structure. While structures are typically designed based on a known initial stiffness of the materials making up the structure, various factors can cause the materials to lose stiffness. Stress, fatigue, and environmental attack such as thermal and/or oxidation processes are just a few of the mechanisms by which a material can be degraded in terms of material stiffness. Fiber/matrix composite materials are particularly susceptible to stiffness degradation, chiefly through a process known as micro-cracking in which microscopic cracks develop in the matrix material that binds the fibers together. Such micro-cracking can cause deleterious changes in mechanical properties, stress concentration, and redistribution within the composite material, which in turn can lead to performance degradation, delamination, and fiber damage. It is difficult, however, to detect micro-cracking using the types of nondestructive testing methods that heretofore have been available.
Prior to the present invention, there was no known nondestructive testing device suitable for use in the field, such as a hand-held device, for quantitatively determining changes in stiffness of a plate such as a composite laminate plate along an in-plane direction of the plate. The prior art teaches various methods for determining stiffness of isotropic materials using ultrasonic wave propagation through the material. For example, U.S. Pat. No. 5,741,971 to Lacy discloses a method for nondestructively measuring a Young's modulus of a bulk isotropic material, such as cements or completion gels used in the petroleum industry for interzone isolation and fracture containment in drilling operations. The method involves using the through-transmission technique in which an ultrasonic transducer is disposed adjacent one end of a sample slug of the bulk material and another ultrasonic transducer is disposed adjacent an opposite end of the sample. The length of the sample between the two transducers is known. An ultrasonic pulse is generated by one of the transducers so as to cause an ultrasonic compression or longitudinal wave to be propagated beginning at one end of the sample, and the other transducer detects the wave when it arrives at the opposite end of the sample. The elapsed time between initiation of the wave at one end of the sample and arrival of the wave at the other end of the sample is measured. Based on this time and the known length of the sample, a velocity of the wave through the sample is calculated. A Young's modulus for the material is then calculated based on the wave velocity and the known density and Poisson's ratio of the material. The method of Lacy and the theory behind it are applicable only to isotropic materials. Lacy's method requires placing transducers on two opposite sides of the sample, and thus would be difficult to apply to in-situ testing of a structure where it may be difficult or impossible to access both sides of the structure. Even if both sides of the structure could be accessed, the through-transmission technique of Lacy still cannot give a measurement of Young's modulus in an in-plane direction, but can only provide an indication of stiffness in the thickness direction, which is the less interesting of the two directions.
U.S. Pat. No. 5,154,081 to Thompson et al. discloses a method for ultrasonic measurement of material properties for metal plates, involving using two transducers and a receiver arranged non-colinearly on one side of the plate. The two transducers generate Lamb waves that propagate along two different directions to the receiver. Based on differences in calculated velocities of the two Lamb waves, Thompson deduces material properties such as grain orientation and stress. The method is applicable only to metals, and does not provide a material stiffness measurement.
There has been a need, therefore, for a nondestructive method and apparatus for measuring in-plane stiffness properties of plates including homogeneous isotropic plates and composite laminate plates. Additionally, there has been a need for such a method and apparatus that can be used for in-situ examination of a plate where it may not be possible to access both sides of the plate.